Jet lag, also jetlag or jet-lag, is a physiological condition which is a consequence of alterations to the circadian rhythm. Such alterations result from shift work, daylight saving time, altered day length, or as the name implies, transmeridian travel as on a jet plane. They are known as desynchronosis, dysrhythmia, dyschrony, jet lag, or jet syndrome. The condition is generally believed to be the result of disruption of the "light/dark" cycle that entrains the body's circadian rhythm. It can be exacerbated by environmental factors.

The maximum jet lag a person can experience is 12 hours. If the difference in time between two places is greater than 12 hours, subtract that number from 24. For example, there is a 16 hour time difference between Los Angeles (standard time) and Hong Kong. Thus, 24 − 16 = 8 hours of jet lag. The person will incur the same amount of jet lag as someone traveling between London and Los Angeles, where the actual time difference is 8 hours. Disregard the difference in date if flying across the International Date Line.

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When traveling across a number of time zones, the body clock goes out of sync with the destination time, and so it experiences daylight and darkness contrary to the rhythms to which it has grown accustomed.[1] The body's natural pattern therefore becomes upset as the rhythms that dictate times for eating and sleeping no longer correspond to the environment of the destination.

Jet lag occurs because the body cannot automatically realign these rhythms. The speed at which the body readjusts itself to new daylight, darkness hours, and eating and sleeping patterns is entirely dependent upon the individual. Thus, while it may take a few days for some people to readjust to a new time zone, others seem to experience little disruption to their body's natural sleeping pattern.

The symptoms of jet lag can be quite varied, though on the whole, an individual may experience the following[2]:

Dr Neil Stanley, past chairman of the British Sleep Society, sounded (4/2006) a note of warning over jet lag due to large time differences. He said: [1]

If you are frequently changing time zone or working long hours or shifts, you do start working at only 60 to 70% of your potential. You lose concentration, you lose judgement, you lose reaction so [e.g.] as a politician you are not going to be on top of your game, to be honest.

The condition is not linked to the length of flight, but to the transmeridian (i.e. east-west) distance traveled. Hence a ten-hour flight between Frankfurt and Johannesburg (going south, staying roughly on the same meridian) is less inducive of jet lag than a five-hour flight between New York and Los Angeles (going west). There seems to be some evidence that traveling west to east is more disruptive, or runs counter to the circadian rhythm. Different individuals may find one adjustment easier than the other. Note that the difference between the two directions decreases as the destination approaches the other side of the Earth (eg from the UK to New Zealand) which 'reverses day and night.' While there is a big difference between a -6 and a +6 hours jet lag, there is relatively small difference between -11 and +11 hours.

Simple prevention can be good sleep aboard and lots to drink (but not coffee or alcohol) to reduce the effects of dehydration on the body, caused by the dry air at altitude and the interruption of regular eating/drinking patterns. Seasoned travelers set their clock to the destination time zone as soon as practical and join the new rhythm. Exposure to sunlight may be a factor to reset the body's clock. For people who don't fly multiple times per week, it can be an effective non-drug remedy to skip sleep entirely for one night and one day and then go to bed at a normal destination-area bedtime on the next day.

Recent research shows that the pinealhormonemelatonin may reduce the effects of jet lag. Studies have not identified side effects from such short-term use. [2] Many products on the market claim to treat the effects of jet lag. Since the experience of jet lag varies among different individuals, it is difficult to assess the efficacy of any single remedy. In addition, most chemical and herbal remedies are not tested or approved by the Food and Drug Administration, so their safety has not been scientifically established.

Acupuncture or acupressure is also a common treatment for jet lag. Whereas various parts of the body work optimally at different periods throughout the day, pressure points are used to stimulate the nerves corresponding to each period at the new location's corresponding time. For example, if a certain organ "turns on" when you wake up, an acupuncturist/acupressurist would advocate stimulating that organ at 7 a.m. each morning once you arrive at the new location. Normally there is a whole series of these points, to be tapped with the patient's finger, at different times throughout the day in order to "trick" the body into thinking it's that time of day.

The condition of jet lag generally lasts a few days or more, and medical experts have deemed that a recovery rate of "one day per time zone" is a fair guideline. Sleep, relaxation, moderate exercise and sensible diet seem to be the simplest recovery agents.